The invention is based on a brake booster for a pressure fluid actuated and in particular hydraulic brake system having anti-skid control (ABS) and traction control (ASR), in motor vehicles, of the type defined hereinafter.
In a known brake booster of this type (German Patent Document 36 29 776 A1), the additional piston for the traction control function is disposed between the pressure rod of the brake piston of the master brake cylinder of the brake system, in a housing chamber separate from the servo piston. In the housing chamber, the additional piston defines a pressure fluid chamber that communicates via a housing connection with the valve assembly comprising two 2/2-way magnetic valves. The pressure rod, which is in one piece with the servo piston, rests on the piston face of the additional piston acted upon by the pressure in the pressure fluid chamber, and the additional piston in turn acts with an integral piston rod upon the brake piston in the master brake cylinder. Upon actuation of the brake pedal, the servo piston, acted upon by pressure fluid via a valve opened by the brake pedal, displaces the additional piston via the pressure rod, and the additional piston via its piston rod displaces the brake piston in the master brake cylinder; as a result, a brake pressure is built up in the work chambers of the master brake cylinder and reaches the wheel brakes, via separate brake circuits. In traction control operation, via one of the two 2/2-way magnetic valves, pressure fluid that is at reservoir pressure is fed into the pressure fluid chamber that acts upon the additional piston. With the servo piston stationary, the additional piston is displaced and in turn displaces the brake piston in the master brake cylinder, so that once again a brake pressure is built up in the master brake cylinder. The magnetic valves in the brake circuits between the master brake cylinder and the wheel brakes that are present for ABS control are simultaneously triggered by the traction control apparatus, in such a way that only the wheel at which a loss of traction occurs, which in other words is spinning, is subjected to the brake pressure.
A brake booster of this kind has a relatively long axial structure, because of the need for an additional piston for the traction control function. The brake piston to be jointly displaced upon each brake pedal actuation is sealed off from the housing chamber by a sealing ring, which necessitates increased force for displacing the brake pedal. Moreover, a restoring spring must be provided between the brake pedal that controls the servo piston and the brake booster housing, to prevent the brake pedal from collapsing upon being actuated during or immediately after traction control operation. This restoring spring further increases the structural length of the brake booster and means that a higher actuating force is required for the brake pedal.